The present invention relates to a telephoto zoom lens having a half viewing angle of 15.degree. to 6.degree. and a zoom ratio of 2.5. This type of zoom lens is very popular and well known among photographic lenses for 35 mm cameras.
The prior art zoom lenses of this type are divided into the following types.
(i) A first lens type in which, although the focal length at the telephoto end to the length from the ratio of the front end of the lens to the focal plane, that is, the telephoto ratio, is greater than 1, the number of the structural lens elements is about twelve which is small.
(ii) A second lens type in which the telephoto ratio is greater than 1 and the number of lens elements is about fifteen.
(iii) A third lens type in which, although the telephoto ratio is smaller than 1, the number of the structural lens elements is greater than fifteen.
(iv) A fourth lens type in which the telephoto ratio is smaller than 1 and the number of lens elements is small but the zoom ratio is smaller than two or approximately two.
(v) A fifth lens type in which the telephoto ratio is smaller than 1, the number of lens elements is small and the zoom ratio is approximately 2.5 but the curvature of field is remarkably varied in response to variation of the focal length.